A centrifugal compressor is often provided with an outlet diffusor, which may be of the passage or of the vane type. A common example of the first type is the so called tubular passage diffusor, which basically consists of a number of inclined, radial passages formed in a plane disc, for instance by boring or by milling.
Most tubular diffusors are expensive to manufacture due to the high precision necessary, and it is very difficult to machine the substantially edge-shaped portion between the entrances to two adjacent passages. The diffusor, further, is often manufactured from light metal, so sand and other foreign matter passing through the filter, usually located at the entrance to the compressor, will rapidly wear down these edge-shaped portions. If the diffusor is produced as an integral unit, repairs are impossible. If, on the other hand, the diffusor is composed of a number of separate components having thin sections and limited sealing areas it will be difficult to prevent leakage transversely to the main direction of flow at these edge-shaped portions. With tubular diffusors of known type, finally, it has not been possible, during actual use, to perform any adjustment of the passages and of the flow characteristics in order to influence surge or choking, respectively, as well as of the efficiency between these two border values.